As is well known, projection lens systems are used to form an image of an object on a viewing screen. The basic structure of a projection lens system is shown in FIG. 8, wherein 10 is a light source (e.g., a tungsten-halogen lamp), 12 is illumination optics which forms an image of the light source (hereinafter referred to as the "output" of the illumination system), 14 is the object which is to be projected (e.g., a matrix of on and off pixels of a LCD panel), and 13 is a projection lens system which forms an enlarged image of object 14 on viewing screen 16.
Viewing screen 16 can be viewed from the front or the back depending upon the particular application of the projection lens system. Also, instead of being viewed, the image can be recorded on a recording medium, e.g., film, in such applications as a photographic enlarger.
For many applications, the light source, the illumination optics, and the location and size of the both the object and image are fixed. For a variety of other applications, however, it is desirable to be able to vary the size and location of the image. In particular, projection lens systems often need to be used with different size screens or in rooms having different dimensions. The parameters of this problem are often expressed in terms of providing variable image distance to image width ratios, which typically run in the range from 7:1 to 1.5:1 (hereinafter the "ID-IW ratio").
In the past, various approaches have been used to vary the ID-IW ratio. The most basic approach has been to use a set of interchangeable fixed focal length projection lens systems with a particular member of the set being chosen to match the ID/IW value of a particular room/viewing screen configuration. The disadvantages of this approach include the facts that only a finite number of ID-IW ratios can be satisfied so that in general the image size is smaller than the projection screen, the setup time for the system is often significant, and the number of components which must be transported and stored is large.
To avoid these problems, zoom projection lens systems have been developed in the art. See, for example, U.S. Pat. No. 3,920,315. These lens systems have followed zoom lens technology developed in the area of "taking" or "objective" lenses, e.g., camera lenses. Examples of such zoom taking lenses can be found in, for example, E. Betensky, "Zoom Lens Principles and Types", SPIE, Vol. CR41, Warren J. Smith, Editor, 1992.
The zoom projection lens systems which have followed the taking lens approach have employed a zooming unit, a compensating unit, a focusing unit which has been either separate from or part of the compensating unit, and a fixed unit containing an aperture stop. As a result, these zoom projection lens systems have been highly complex containing many lens elements. Also, for low ID/IW values, these design forms would require excessively large diameters for the lens elements which would significantly increase manufacturing costs.
Zoom lens objectives have been developed for camera applications which employ a moving physical aperture stop. Such a stop can be used to minimize both element size and aberration variation during zooming. Reduced aberration variation, in turn, reduces lens complexity. By means of this approach, significant simplification in terms of both economy of elements and motions of elements has been achieved. See U.S. Pat. No. 4,838,669. A related approach has employed multiple physical stops at different locations in the zoom lens system, with different stops controlling the aperture and vignetting of the system as zooming takes place. See U.S. Pat. No. 4,749,265.
As discussed in detail below, in accordance with the invention, it has been determined that the moving aperture stop approach is not suitable for zoom projection lens systems. Specifically, such a stop generally results in a moving entrance pupil which leads to a variety of problems when used with fixed illumination optics having an output at a fixed location, the most serious of which is that small ID/IW values cannot be achieved. Although multiple physical aperture stops can in theory be used to address the moving entrance pupil problem, the result of this approach would be an unnecessarily complex and expensive zoom projection lens system.
The problems resulting from a zoom projection lens system having a moving physical aperture stop can be addressed by using illumination optics having a moveable output location. However, this simply transfers the problem to the illumination portion of the overall projection system making that portion complex. In addition, illumination systems having moveable outputs are inefficient in terms of illumination at the viewing screen per unit of energy consumed, i.e., higher wattage lamps must be used to achieve the same level of screen illumination.
Zoom projection lens systems specifically designed for use with LCD panels have been disclosed in Japanese Patent Publications Nos. 4-172416, 4-83215, and 3-293612. The '416 publication uses the classical approach of a fixed aperture stop and thus the resulting lens system is highly complex including four units and ten elements. The '215 publication has a moving aperture stop resulting in a moving entrance pupil which limits its ability to achieve low ID/IW values. The '612 publication similarly has a moving aperture stop and thus entrance pupil which again limits the range of ID/IW values. Moreover, the lens systems of this publication exhibit variations in light level at the screen as the lens is zoomed.
Projection systems which employ LCD panels present special problems for a zoom projection lens system. For example, beam splitting optics are often used with LCD panels so that three colors can be projected using one projection lens system. For these applications, there needs to be a large space between the object and the first lens element of the zoom projection lens system.
The corresponding problem for a taking or objective lens involves creating a large back-focal-length to focal-length ratio. For wide angle taking lenses, an inverted telephoto design composed of a negative first unit followed by a positive second unit containing an aperture stop can be used for this purpose. Such lenses, however, always have a moving exit pupil, or entrance pupil in projection lens terminology. This means that conventional inverted telephoto zoom lenses cannot be directly applied to the problem of providing a large space between the object and the first element of a zoom projection lens system.
Another problem associated with zoom projection lens systems which are to be used with LCD panels, as well as in other applications, is that in some cases it is necessary for the entrance pupil to be at infinity or at least at a great distance from the object, a condition referred to as telecentric. This is either to minimize the angle of the principal ray at the object, or to minimize the change of magnification for an out of focus condition. To achieve this condition, the aperture stop is usually placed at the back focus position of the rear lens unit. Depending upon the size of the object, and the distance from the object to the first lens surface, the back focus position could be a considerable distance from the rear lens unit, thus requiring a large physical size for the lens barrel. In the case of a zoom projection lens system designed in accordance with prior art approaches, this problem is exacerbated because the moving elements would have to be on the image side of the aperture stop.